This renewal application deals primarily with the study of principles of organization, plasticity, and development of CNS centers that are provided with microneurons. The research is centered on the cerebellum and the cochlear nuclear complex. A feasibility study will be initiated in the third year of the program for determining if neonatal hyperthyroidism has measurable effects on the formation and development of olfactory bulb microneurons as it does in the cerebellar and hippocampal cortices. The program intends to refine qualitative and quantitative data on the cerebellar circuitry (the mossy-fiber-granule cell-Purkinje cell-cerebellar nuclear neuron system) using adult, developing and experimental animals, including certain murine mutants). A second major goal is to ascertain whether the granule cells in the mammalian cochlear nuclei constitute a system analogous to that of the crebellar cortex, with input to the principal neuron of the DCN, the pyramidal cell. A standard battery of procedures including Nissl section, Golgi impregnations, Nauta methods, horseradish peroxidase labeling, autoradiography, AChE staining, electron microscopic methods, freeze-fracturing, morphometry, microsurgery, already in use in the laboratory, are employed as appropriate for the individual projects. Experimental animals: chickens, rodents and cats (a few monkeys used for other experiments in other laboratories will also be utilized). The proposed studies are of significance for understanding brain structure and plasticity.